A considerable amount of effort has been expended in the development of apparatus which is adapted to receive sticks or rods of hot-melt adhesive, heat them to melting, and allow their controlled application to the surfaces to be bonded by the adhesive. In general, the problems encountered by the prior art inventors, and evidenced by the apparatus they have developed, include the mechanical problems associated with achieving a compact apparatus, achieving good adhesive-heating capacity, and achieving a dependable advancement of the unmelted adhesive through the gun without fouling of the gun by the adhesive.
Among the tools disclosed in the prior art is that disclosed in U.S. Pat. No. 3,587,930. This device is adapted to receive a stick of glue which is pushed from its rearward position, by a piston, towards a heating zone in which the glue is melted. There is a small and continuous air stream bled around the adhesive stick in an attempt to keep the stick from melting prematurely.
Other devices are disclosed in all of the following U.S. Pat. Nos. 2,742,143; 2,995,159; 3,314,574; 3,774,817, 3,612,357; 3,587,930; and 3,743,142. In general, none of these tools achieve the desirable positive advancement of the adhesive without some substantial sacrifice in terms of mechanical simplicity or dispensing capacity. As will be described in some detail below, one of the problems of the earlier apparatus relates to the means whereby the apparatus grips the adhesive. In some cases, (e.g. U.S. Pat. Nos. 2,995,159 and 3,612,357) the glue stick must be of a particular shape. In other cases (e.g. U.S. Pat. Nos. 2,742,142; 3,774,817; and 3,314,574) the selective engagement of only part of the adhesive rod presents a problem in the uniform advancement of the adhesive stick which, depending upon the precise adhesive being used, may have substantial plasticity in the gripping area. Some U.S. Pat. Nos. (e.g. 3,743,142 and 2,437,263) disclose adhesive advancement means either engaging or spaced around a substantial segment of the circumference of the adhesive stick. But these advancement means do not adequately grip into a plastic adhesive rod.
Other apparatus comprises using constant pressure on the adhesive and consequently is susceptible to fouling by unwanted extrusion of the adhesive into parts of the gun where it is troublesome.
Thus, in general, there has been a need to provide a simple, more compact dispensing apparatus adapted to achieve a reliable and repeatable advance of hot melt adhesives.
It is to be noted that the comments on the problems of the prior art are made in hindsight, i.e., in view of the claimed invention. It is not to be inferred from this discussion that all of these problems were recognized in the prior art.